Soft X-ray sources of high intensity are applied in many fields, for instance surface physics, materials testing, crystal analysis, atomic physics, lithography and microscopy. Conventional soft X-ray sources, which utilise an electron beam towards an anode, generate a relatively low X-ray intensity. Large facilities, such as synchrotron light sources, produce a high average power. However, there are many applications that require compact, small-scale systems which produce a relatively high average power. Compact and more inexpensive systems yield better accessibility to the applied user and thus are of potentially greater value to science and society. An example of an application of particular importance is X-ray lithography.
Ever since the 1960s, the size of the structures that constitute the basis of integrated electronic circuits has decreased continuously. The advantage thereof is faster and more complicated circuits needing less power. At present, photolithography is used to industrially produce such circuits having a line width of about 0.35 .mu.m. This technique can be expected to be applicable down to about 0.18 .mu.m. In order to further reduce the line width, other methods will probably be necessary, of which X-ray lithography is a potentially interesting candidate. X-ray lithography can be implemented in two ways: Projection lithography, where use. is made of a reducing extreme ultraviolet (EUV) objective system in the wavelength range around 10-20 nm (see for instance Extreme Ultraviolet Lithography, Eds. Zernike and Attwood, Optical Soc. America Vol. 23 [Washington, D.C., 1994]) and proximity lithography, which is carried out in the wavelength range 0.8-1.7 nm (see for instance Maldonado, X-ray Lithography, J. Electronic Materials 19, 699 [1990]). The present invention relates to a new type of X-ray source, whose immediate field of application is proximity lithography. However, the invention can also be used in other wavelength ranges and fields of applications, such as EUV lithography, microscopy, materials science.
Laser-produced plasma (LPP) is an attractive compact soft X-ray source owing to its small size, high luminous intensity and great spatial stability. Here a target is illuminated by a pulsed laser beam, thereby to form an X-ray-emitting plasma. However, LPP which uses conventional solid targets suffers from serious drawbacks, inter alia, emission of small particles, atoms and ions (debris) which coat and destroy, for example, sensitive X-ray optical systems or lithographic masks arranged close to the plasma. This technique is disclosed in, for instance, WO94/26080.
This drawback can be eliminated by using small and spatially well-defined liquid droplets as target and irradiating them with a pulsed laser beam as disclosed by Rymell and Hertz, Opt. Commun. 103, 105 (1993). According to this publication, the droplets are generated by forming a jet of liquid by urging the pressurised liquid through a small nozzle, which is vibrated piezoelectrically. This droplet-generating method is described in e.g. U.S. Pat. No. 3,416,153 and in Heinzl and Hertz, Advances in Electronics and Electron Physics 65, 91 (1985). This results in very small and spatially well-defined droplets. In addition to eliminating debris, this compact X-ray source gives an excellent geometric access, a possibility of long-term operation without interruption since new target material is continuously supplied, and a possibility of a high average X-ray power by using lasers having a high repetition rate. A similar technique is disclosed by, for instance, Hertz et al, in Applications of Laser Plasma Radiation II, M. C. Richardsson, Ed., SPIE Vol. 2523 (1995), pp 88-93; EP-A-0 186 491; Rymell et al, Appl. Phys. Lett. 66, 20 (1995); Rymell et al, Appl. Phys. Lett 66, 2625 (1995); and U.S. Pat. No. 5,459,771.
A drawback of this technique is however that all liquids cannot form sufficiently spatially stable microscopic droplets, and therefore it will be difficult to guide the laser light so as to irradiate the microscopic droplets. Moreover, there are also for suitable liquids slow drifts in droplet position relative to the focus of the laser beam, which results in the synchronization of the laser plasma production requiring temporal adjustment.